WO 2012/092911 A1 describes a method for detecting rain comprising a camera and a lighting source. The camera is disposed behind a pane, in particular in the interior of a vehicle behind a windscreen and focused onto a remote region that lies in front of the pane. The lighting source for generating at least one light beam that is directed at the pane directs the at least one light beam towards the pane such that at least one beam that is reflected from the outer face of the pane impinges on the camera. The light quantity of the at least one beam that impinges on the camera can be measured by the camera.
In order to detect raindrops also at night, it is described in WO 2010/072198 A1 to couple light via a coupling element into the windscreen and to guide it via total reflection in the pane. By a decoupling element the totally reflected light is decoupled in the direction of the camera.
When there are water drops on the windscreen, a part of the light is decoupled and is no longer totally reflected to the decoupling element. It is again disadvantageous here that for each modified pane inclination the integrated camera lighting unit must be mechanically adapted to the modified installation condition.
In known active methods for rain detection a light source specifically required for this purpose is used, by which the reduction of the reflected light intensity is measured and used for rain detection. The projected light, however, encloses only a relatively small surface on the windscreen, which is used for the detection. Raindrops outside the detection surface remain undetected due to the light sources or LED-based detection method. In case of light or moderate rain it can happen that the windscreen is wetted uniformly with drops, whereas no drops fall onto the position of the detection surface. For a stable rain detection, in particular with light precipitation it is, therefore, desirable to increase the detection surface.